Micro-fuses are widely used in electronic printed circuits or the so-called printed circuit boards (PCB) in various electrical and electronic equipment. These micro-fuses, which are typically 2.5 mm.times.8 mm, are presently made by a rather cumbersome procedure. According to the conventional method of manufacturing these fuses, a long, generally cylindrical glass or ceramic tube is first cut to the desired length. A fusible element is then stretched diagonally between the ends of the tube and the ends are prefilled with solder. Lead wires are inserted through a perforation at each end cap and are secured to the inside surface of each respective cap. Thus, when the caps are heated, the solder melts and electrical contact is established between the fusible element and the lead wires which are connected to a power source.
In order to insure electrical insulation, the fuse casing is either coated with an insulative material, usually epoxy resin, or it is covered by a thermal shrink insulating tube.
Micro-fuses made according to such conventional methods have several drawbacks and limitations which are inherent in their structure and the method by which they are fabricated. For example, it is generally recognized that it is not desirable to attach, usually by solder, the lead wires to the outside end of the casing because lead wires attached in this manner cannot withstand the tensile strengths to which they are usually subjected due to industrial requirements. In addition, soldering of the lead wires to the outside casing is difficult or impractical. Accordingly, the preferred practice is to solder the lead wires interiorly of the casing, i.e., to the inside surface of the end caps of the fuse. This, however, requires that the fusible element be first passed through the tube and soldered to the inside of the end caps in order to maintain the fusible element at a fixed length while stretched diagonally through the tube. As a practical matter, however, it is difficult to stretch the fusible element diagonally and retain it at a fixed and invariable length because the proper setting of the fusible element cannot be found in cylindrical tubes with generally circular ends. Also, while it is the general practice to hold the fusible element manually and to keep it taut in the stretched position until the melted solder solidifies, this procedure becomes impractical when the fusible element is to be soldered interiorly of the casing. Consequently, before solidification of the solder melt, the fusible element tends to slacken, hence resulting in unstable electrical characteristics due to variations in the length of the fusible element.
Another drawback of conventional micro-fuses results from inadequate bonding of the end caps to the tube casing. Since the solder usually contains a flux (chiefly made of rosin), after soldering, the bond strength between the end caps and the casing is insufficient and cannot retain the necessary mechanical integrity of the structure.
Also, since the end caps of a conventional micro-fuse with lead wire are exposed, they must be electrically insulated. This protection has been afforded by coating with epoxy resin or providing the casing with a thermal shrink insulating tubing. However, epoxy resin coatings and such protective coverings tend to deteriorate at temperatures of 120.degree. C. to 200.degree. C. and, therefore, the resulting fuse cannot be satisfactorily used under such conditions. Moreover, the usual method of marking the rated current and voltage on the fuse is not practical for such coated or protected fuses, making it necessary to replace such marking with color code or label marks, both of which involve cumbersome procedure.
Coating of the micro-fuse with epoxy resin, and the provision of a protective insulative covering, involves additional, and often time consuming and expensive steps in the manufacture of these fuses. Needless to say that the productivity will therefore be considerably reduced.
With the ever-increasing industrial demand for high speed manufacture of micro-fuses which employ extremely fine and small fusible elements, the conventional method of micro-fuse manufacture offers limited capacity to fullfill this objective. In addition, micro-fuses made by the conventional method often fail to meet the stringent industrial requirements.
Accordingly, it is an object of this invention to provide a micro-fuse which, due to unique construction and configuration of its component parts, can be assembled and manufactured efficiently.
It is a further object of this invention to manufacture such micro-fuses by an improved method which simplifies the manufacturing and production procedures for these fuses.
The foregoing and other objects of the present invention will be more clearly comprehended from the following detailed description of the invention and the accompanying drawings.